Archive | April, 2012

Mid-April Longer Term Thoughts & Analysis…

15 Apr

Spring is often a season with extremes.  Many areas across the Northern Hemisphere under-go changes on quite short time scales as we continue to see the influence of an increasingly strong sunshine.  However, the remnants of winter, characterised by a lingering cold pool of air to the north of the UK, can still make an appearance.  Clearly the last 4 to 6 weeks across the UK have produced extremes.  From balmy temperatures above 20C in mid to late March, to being replaced by near blizzard conditions in places a week or so later.  April is now certainly producing the classic ‘April showers’ type weather as well as low pressure and unsettled conditions generally continue to dominate the weather with some heavy and thundery downpours over the last week or so.

There has been some recent discussions over the longer term outlook covering the second half of April and the main emphasis is on unsettled conditions, as low pressure dominates the weather.  The primary question is; why is the weather likely to be persistently unsettled?…

The answer, as ever, lies in the expected broader scale weather patterns including the position and location of larger scale troughs and ridges (jet stream) and the below image highlights what is expected during the second half of April overall;

Now when it comes to looking at the medium and particularly the longer term expected weather patterns, it is of greater importance to attempt to find the broader scale pattern, rather than the detail.  The above image highlights the fact that throughout the rest of April a large and slow moving trough is expected across the UK and other North-western areas of Europe. In broader terms a ‘trough’ is indicative of unsettled conditions and low pressure, bringing predominantly unsettled conditions with rain, showers and with temperatures generally average at best.  In comparison a ridge in the jet stream, which is expected to be the case across far Eastern areas of Europe and into Russian will clearly signal the opposite and that will be surface high pressure and settled conditions.

Now using a few forecast charts to illustrate this, you can see that on both the GFS and ECM ENS (ENS = Ensemble) charts at 240hr, low pressure is dominant across the UK in association with a marked upper trough, whilst pressure is generally higher across Eastern Europe;

Now if you attempt to compare the actual forecast charts, to the first image I have constructed, you can see that at this particular time frame (240hr 25th April 12) the pattern matches very well indeed.  So, in summary, if you are after a rapid return to more settled and warmer conditions like what was experienced in March, or even just a trend towards more settled and drier conditions, then that is highly unlikely.  Clearly the second half of April in meteorological terms is a long time and things can change, but it certainly looks as though that the second half of April will produce unsettled conditions with rain, showers, some strong winds and predominantly cool temperatures across the UK…

…however, good news for those in the hosepipe ban regions!

Regards to all,

M.

 

Atmospheric Soundings & Convective Activity…

12 Apr

As many will have noticed over the last few days there have been some very heavy showers evident during the afternoon  period, with some locally torrential downpours with hail and even some scattered thunderstorms as well.  In a previous blog I placed some emphasis on how atmospheric soundings can be used to gauge cloud amounts in anticyclonic (high pressure) conditions.  Well, equally and also very importantly, atmospheric soundings come into their own during times of instability and particularly between the months of April and September…

The aim of this particular blog is to shed some light on how atmospheric soundings, which are freely available on the internet, can be used to highlight the risk of instability which can lead to heavy showers and thunderstorms in the coming weeks and months in particular.

The first image of interest here is an atmospheric sounding from Leeds at 1600BST on Wednesday 11th April.  I have used this particular sounding as it was one of the best to highlight instability.  Now as a quick recap, the red line represents the air temperature up through the atmosphere, whilst the blue line is the dew point temperature.  Now when the air becomes unstable, on these particular charts, an additional line appears and in this instance this is the dotted line I have highlighted within the image.  In essence what this line represents is that it signifies that the air mass is unstable and that the air can freely rise.  Clearly when air rises it brings the development of clouds and if the air can rise enough it can lead to significant cloud development with large towering cumulus clouds and cumulonimbus clouds (thunder clouds).

The next image, highlights more specific and important features;

The image should highlight and explain some of the primary features that are of interest.  I have overlaid the picture of a cumulonimbus cloud which places emphasis on the level of instability present within this atmospheric sounding and that with clouds potentially rising up to 24,000ft easily leads to the development of cumulonimbus clouds and hence, intense and perhaps thundery downpours at  the surface.

Now moving on to take a look at one primary important variable and that is CAPE.  This stands of Convective Available Potential Energy and is a measure of the instability present within the atmosphere.  CAPE is derived from the space present between the temperature line (red line) and the dotted line I have highlighted on the images.  The larger this space the greater the CAPE value is and hence the greater the instability also is.  You will note that on this specific image the CAPE value is provided at the top beneath the date and time.  In this instance the CAPE value is 394j/kg (joules per kilogram), which is a reasonable amount of CAPE.

As a general rule of thumb from a UK point of view the following should give an indication as to the risk of heavy and perhaps thundery downpours;

CAPE VALUES: 

100-300 = Marginally unstable, risk of some heavy showers, but thunderstorms generally isolated or scattered.

300-600 = Moderately unstable, heavy showers/downpours a distinct possibility, thunderstorms also probable.

600-900 = Highly unstable, heavy showers, downpours and thunderstorms a significant likelihood.

>900-1000 = Extremely unstable, torrential, thundery downpours a high probability, risk of severe convective weather.

CAPE values are a changing variable and for example across parts of the USA CAPE values can reach and exceed 2500j/kg, which is clearly an extreme level of instability and because of many variables those kinds of values are never achieved in the UK.  But from a UK’s perspective the above values should give a reasonable indication as to the level of instability that is forecast.

So, in summary, if you hear that thunderstorms are forecast and see atmospheric soundings similar to the ones used within this blog, along with large CAPE values then envisage the risk of heavy and perhaps thundery downpours.  The forecast soundings used within this blog can be found at the following website;

http://rasp.inn.leedsmet.ac.uk/RASPtable.html

Regards to all,

M.